Recombination between divergent genes and genomes occurs commonly in nature and generates new gene combinations that appear to be important in the evolution of virulence, as well as resistance to antibiotics and disease. However, we know surprisingly little about the frequency with which wide recombination or hybridization contributes to adaptive evolution or the evolutionary conditions required for the establishment of new hybrid gene combinations. Given the ever-increasing opportunities for contact and gene exchange between previously isolated organisms, and the possible negative consequence of these interactions for human health, it is important that we understand the principles that govern the outcomes of hybridization. The proposed research will address fundamental questions about the role of hybridization in evolution, the limits to ecological divergence, the efficacy of different kinds of chromosomal rearrangements as reproductive barriers, the proportion of genes under positive selection during speciation, the reconstruction of reticulate phylogenies, and the tempo of hybrid speciation. The research will be conducted using the wild, annual sunflowers (Helianthus) because this group is widely cited as a model system for the study of interspecific hybridization and its consequences. The experimental plan employs a broad array of approaches, ranging from classic selection experiments comparing the response to selection of hybrid and pure parental populations, to candidate gene mapping, to single nucleotide polymorphism analyses that will assess the scale of recombination between the parental genomes in hybrid lineages. The proposed work represents an important step toward understanding the contribution of hybridization to adaptive evolution and speciation.